1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having a DC-DC converter of a charge pump circuit type and configured so as to use its output as a reference voltage for an amplifier circuit.
2. Description of Related Art
Headphone amplifiers, video drivers, and other semiconductor circuits conventionally have required a ground potential to be used as the bias voltage of output signals, but since the ground potential was used as the reference potential of the circuits, the bias voltage of the output signals was positive. Accordingly, a bias voltage had to be converted to a ground potential by connecting a decoupling capacitor between the output and the load. However, some semiconductor circuits that have become popular in recent years have a negative power supply built into the semiconductor circuits to make the reference voltage negative, thereby permitting use of the bias voltage of the output signals as the ground potential without conversion. In such semiconductor circuits, an inverting charge pump circuit is used as a means for generating a negative power supply.
An example of such a semiconductor circuit with a built-in negative power supply has been disclosed, for instance, in the specification of U.S. Pat. No. 5,289,137. Below, the semiconductor circuit disclosed in the specification of U.S. Pat. No. 5,289,137 will be explained with reference to FIG. 3.
In FIG. 3, reference numeral 21 designates a semiconductor chip, which has an inverting charge pump circuit 22 and an amplifier circuit 23 built into it. A power-supply voltage (VCC) is supplied from an external power supply 24 to the semiconductor chip 21, and a flying capacitor 25 and output capacitor 26 are connected to the charge pump circuit 22.
The inverting charge pump circuit 22 uses the external power supply 24 as its power supply and outputs a negative voltage obtained by inverting the power-supply voltage (VCC) obtained from the external power supply 24 to a charge pump output terminal 27. At such time, the absolute value of the charge pump negative output voltage reaches a value (−VCC) nearly equal to the power-supply voltage obtained from the external power supply 24. In operation, the amplifier circuit 23 uses the charge pump negative output voltage (−VCC) as a reference voltage and external power supply 24 as its power supply.
Input signals to the amplifier circuit 23 are inputted via a signal input terminal 28 and output signals from the amplifier circuit 23 are outputted via a signal output terminal 29.
The incorporation of the negative power supply circuit in the semiconductor with a single power supply is made possible by sharing the power supplied by the external power supply 24 between the charge pump circuit 22 and amplifier circuit 23.
However, the problem with the circuit disclosed in the specification of U.S. Pat. No. 5,289,137 is that the maximum value of the voltage of the external power supply 24 has to be limited so as not to exceed the withstand voltage of the charge pump circuit 22 and amplifier circuit 23.
Considering the convenience of assembly in using the semiconductor chip 21, it is undesirable to limit the maximum value of the voltage of the external power supply 24. Although such a problem does not occur if the circuit is composed of elements possessing high withstand voltages, it becomes important to address this problem when using CMOS circuits, which are superior in terms of cost and power savings.
This problem is explained below with reference to FIG. 3. In FIG. 3, the negative output voltage of the charge pump circuit 22 changes in concert with the external power supply 24, such that when the external power supply voltage is +VCC, the charge pump negative output voltage becomes practically equal to −VCC. Therefore, if the voltage of the external power supply 24 increases by ΔVCC, the voltage on the charge pump circuit 22 and amplifier circuit 23 will increase by 2×ΔVCC.
A withstand voltage of the amplifier circuit 23 is limited by a withstand voltage of the MOS transistors composing an inverter 30 of an output stage thereof. When designating this withstand voltage as Vr, the acceptable ΔVCC will be:ΔVCC<(Vr−VCC)/2.
Thus, in comparison with an amplifier using a ground potential as the reference potential, an amplifier using a negative output voltage supplied by a charge pump circuit as a reference voltage has to suppress the amount of increase in the external power supply voltage relative to the usual value.
In addition, if ripples are generated in the external power supply 24, ripples with a 180° phase shift and identical magnitudes as shown in FIG. 4 are superimposed on the negative voltage output of the charge pump circuit 22. For this reason, larger ripples are generated in the output signal of the amplifier circuit 23 driven by those positive and negative voltage, in comparison with amplifiers using the ground potential as the reference potential.